The present invention relates to data communication networks comprising transceiver devices, and particularly to networks permitting a communication of multipoint type in which each transceiver can communicate with each of all the other transceivers on the same network.
The data communication needs brought about by computer systems have caused the development of high speed telecommunications networks, in the Megabauds range, and the common usage of synchronous transmission techniques. In these techniques, the data signal coding scheme usually permits regeneration of a clock signal at the receiving side, which is to say that the transmitted data also carries the sender clock signal. The clock is used by the receiver to sample the incoming binary signal and deserialise the information. In this type of connection, all characters are emitted in continuous sequence without any intermediate pause and the transmission initialisation is generally characterised by the emission of a sequence of binary signals which are used by the receiver to synchronise itself. All transmission interruptions affect the loss of synchronisation of the receiver.
Using these synchronous transmission techniques, networks of multipoint type have been developed where the direct communication from sending to receiving equipment can be achieved. For this purpose, the equipment makes use of methods based on the contention principle, which is to say that each transceiver on the network can transmit when no other communication is in process. In all multipoint contention systems known today, the transmission type is synchronous, high speed, with carrier suppression during transmission pauses, so that all transceivers can identify very rapidly the free/busy state of the network.
French patent No. 2 306478 describes the following device: a message transmission starts with a synchronisation binary signal, and an interruption at any time is interpreted as end of the message, which puts the channel back in the quiescent state. This device can work only with a signal coding scheme which carries the clock of the sender, and the transmission is of synchronous type. The messages contain the addresses of the sender and of the receiver or addressee. In case of data collision on the network, the senders stop transmitting and restart transmission after a waiting time of random duration which, in practice, can be almost null. No collision signal is generated on the network.
European patent application No. 0 023105 also describes a device using the contention principle; in this device, the message is transmitted using a particular coding scheme containing packets of eight binary signals each. The transmission technique is of synchronous type, the message carries the clock, but no destination address is sent so that all transceivers on the network are addressees of all messages. The particular coding scheme enforces a wait of duration equivalent at least to a packet, before transmission can be retried after collision.
British patent application No. 2 013452 describes a device in which the last sender equipment designates the succeeding one; thus the devices are authorised to transmit one after the other in a modifiable succession. This is a technique quite different from the contention technique.
Asynchronous data transmission networks permit only lower speed implementations but are far less expensive. The identification of the availability of an asynchronous network is more delicate; this is why multipoint contention channels have never been developed. As a matter of fact in the asynchronous transmission technique, messages are composed of a sequence of characters separated by time intervals during which the carrier frequency is suppressed. Consequently the absence of the carrier frequency does not guarantee that a message is not being transmitted.
Therefore, in the networks which utilise the asynchronous communication technique, multipoint lines have been implemented where a master device polls the slave device to acquire the information it has to transmit, and selects the slave device to transmit to it the information it needs to receive. This approach leads to a hierarchical network architecture which is rather complex and inflexible.